Thermal lithographic offset printing plates are known in the prior art. For examples, U.S. Pat. Nos. 6,124,425 and 6,177,182 taught to prepare positive and negative working printing plates comprising a radiation sensitive layer that can be imaged with near infrared laser light and developed with aqueous developers.
U.S. Pat. Nos. 6,994,327 and 6,899,994 taught to prepare negative-working thermal lithographic printing plates comprising a hydrophilic polymer under layer and a radiation sensitive imaging layer. These plates can be imaged with near infrared laser light and developed on-press using inks and fountain solutions.
Thermal lithographic printing plates comprising an overcoat layer are also known. For example, U.S. Pat. Nos. 6,482,571 and 6,541,183 taught to prepare negative-working thermal lithographic printing plates comprising a radiation sensitive under layer and an optional water-soluble polyvinyl alcohol overcoat layer. This optional overcoat layer is transparent and insensitive to light. These plates can be imaged with near infrared laser light and developed on-press using inks and fountain solutions.
U.S. Pat. No. 6,846,614 taught to prepare negative-working thermal lithographic printing plates comprising a radiation sensitive layer and water-soluble overcoat layer obtained from a mixture of polyvinyl alcohol and polyvinyl imidazole. This optional overcoat layer is transparent and insensitive to light. These plates can be imaged with near infrared laser light and developed on-press using inks and fountain solutions.
More generally, the prior art teaches to prepare negative-working thermal lithographic offset printing plates comprising a near infrared laser light sensitive layer. This imaging layer typically includes near infrared absorbing compounds, initiators (either radical or cationic), and binder resins and may also include reactive oligomers, colorants, etc. These plates may comprise an overcoat layer which usually protects the near infrared laser light sensitive layers (imaging layer) from either or both oxygen and moisture and thus prevent the background staining and reduction of the imaging speed normally caused by these species in unprotected printing plates.
However, the initiators typically used in the imaging layers are iodonium salts, sulfonium salts, triazine compounds and the like. These initiators are well known to be sensitive to white light. In fact, these initiators react with white light and cause many problems such as (often severe) background staining. This means that the plates cannot be handled or processed under natural light. To partly overcome this problem, the plates are usually handled under “yellow light”, that is white light filtered to remove light with frequencies below about 550 nm.
Another problem with prior art printing plates arises from the fact that they are usually stacked on top of each other during shipping and storage. Since the coatings on these plates are soft and sticky, protective interleaving papers need to be inserted between each of the printing plate to prevent them from scratching and sticking together. This is disadvantageous because the interleaving papers increase costs and must be removed manually prior to imaging (which further add to costs). If the papers are not removed, they will cause paper jams in the automatic plate loading plate-setters used to load the plate in the printing machines.
There thus remains a need for better printing plates that can be handled in white light and that do not require the use of interleaving papers for storage.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.